Many electrical machines having electrical windings have a requirement for high reliability. While this requirement is applicable to many different types of electrical machines, including dynamoelectric machines such as motors or generators, it is especially applicable to motors and generators used in aircraft due to the high costs associated with replacement or repair as well as the need for unfailing operation in flight.
It is well known that, where such high reliability is required, it is desirable to provide means for protecting the electrical machine against damage due to overheating. Typically, such protection means include a temperature sensitive device disposed within the electrical machine to detect a rise in temperature, in conjunction with a thermal protection circuit connected to the temperature sensitive device and configured to cause electrical power to the electrical machine to be interrupted before the electrical machine is adversely affected.
There are applications for electrical machines, such as motors or generators located within passenger compartments of aircraft or within living spaces of a building, where in addition to protecting the electrical machine from damage due to overheating, it is required that the electrical machine be protected in a manner which precludes the formation of smoke, noxious gases, or odors which could be detected by, and lead to anxiety among passengers or crew of the aircraft, or occupants of such a living space.
Such smoke, gases, or odors are typically generated by overheating of materials such as wire insulation, potting compounds, or lubricants utilized within the electrical machine, which typically begin to decompose, oxidize, or outgas when exposed to elevated temperatures. Unfortunately even for the best such materials presently available, the temperature at which detectable levels of smoke, gas, or odor are produced, is considerably lower than maximum safe operating temperatures which are typically considered acceptable in applications where protection of the electrical machine against damage due to overheating is the sole concern.
Accordingly, in designing an electrical machine for use in applications where the generation of smoke, gases, or odors is not permissible, it is imperative that the means for protecting the electrical machine from overheating be capable of detecting an overheat condition and shutting down the electrical machine quickly enough to prevent the electrical machine from reaching a minimum temperature at which detectable amounts of smoke, gas or odor are produced.
Such protection systems have utilized various types of strategically positioned temperature sensitive devices, including thermally actuated fuses or switches, and solid state sensors such as thermistors, which product a change in circuit resistance in response to changes in temperature, to product a signal detectable by the protection circuit. While the exact choice of the type of temperature sensitive device and the location within the electrical machine is largely dictated by design requirements unique to a given electrical machine and application, it is well known that in order to provide optimum protection and to achieve the fastest response time, the temperature sensitive device should ideally be located at a point within the electrical machine where a so-called hot spot is most likely to occur under abnormal operating conditions.
In a dynamoelectric machine, such as a motor or generator, it is further well known that such a hot spot is most likely to occur in end turns of the stator of such a motor or generator. This is due to the fact that a potential overheating problem due to an electrical malfunction, such as a short circuit or locked rotor, will typically manifest itself through a rapid increase in current flow within the electrical machine which in turn results in increased heat generation and rapid rise in the temperature of the stator, as well as the rotor. Experience has shown that, in such an instance, the temperature of the stator end turns will increase more rapidly than other elements within the electric machine due to the fact that the end turns have less mass and are not cooled as effectively as the other elements within the electrical machine.
As a result, it is common practice to include a temperature sensitive device in the form of one or more small thermally actuated switches in the end turns of stator windings, of dynamoelectric machines such as motors or generators. These switches typically include small bimetallic elements configured so as to open and close when a certain temperature is reached. The opening or closing of the switch in turn opens or closes an electrical circuit which is employed with other equipment such as a thermal protection circuit to shut down the dynamoelectric machine before it overheats to the point of damage or generating smoke, etc. Following shut down of the machine, the high temperature gradually dissipates, and ultimately the switch will revert to its original condition, restoring the electric circuit to its former state. This can be used as a means to automatically restore the electrical machine to service.
While this basic approach is conceptually simple, actual implementation is found to be difficult by virtue of the fact that the temperature sensitive device must be installed in a manner which ensures intimate contact with the end turns of the stator in order to achieve adequate protection and fast response times.
Through the years many different variations on the basic approach to providing protection of such electrical machines against damage due to overheating, and methods for improving the response time of such protection, have been developed, as exemplified by U.S. Pat. Nos. 1,947,078; 2,471,840; 3,131,322; 3,200,274; 3,422,313; 4,188,553; 4,571,518; and 4,926,077.
In one commonly utilized variation of the basic approach, the temperature sensitive device is installed directly into the end turns during winding of the stator, and the end turns are subsequently compressed and plastically deformed to obtain contact with the temperature responsive device. While this approach results in reasonably good contact between the end turns and the temperature responsive device, the temperature responsive device and the end turns are prone to damage during fabrication of the stator.
In another approach, heat transfer devices, such as heat sinks, insulators or shields, or pocket devices for holding the temperature sensitive device, are inserted into the stator either during or subsequent to winding of the end turns, and the temperature sensitive device is attached or inserted following completion of winding of the stator. While this approach does provide protection against damage to the thermally sensitive device during fabrication of the stator, the thermal response of such an installation is typically slower than when the temperature sensitive device is in intimate contact with the end turns, due to the increased thermal resistance incurred as a result of addition of the heat transfer or pocket device into the heat transfer path between the end turns and the temperature sensitive device. In addition, with this approach the end turns must typically be compressed or otherwise plastically deformed about the heat transfer or pocket device in order to ensure adequate thermal contact, thereby exposing the end turns to potential damage during fabrication of the stator.
In yet another approach, an installation tool is forced between adjacent end turns to form a pocket which allows insertion of the temperature sensitive device, and following insertion of the temperature sensitive device, the end turns are compressed around the temperature sensitive device and laced in place to provide intimate thermal contact between the end turn and the temperature sensitive device. This approach suffers from a number of problems, including the fact that insulation on the end turns may be nicked or scratched during the process of inserting the sensor, and that successful installation is highly dependant on the skill of the technician. Furthermore, experience has shown that, in order to insert the temperature sensitive device in this manner, the end turns must be plastically deformed, creating the potential for mechanical failure of the end turns if they are subjected to inertial or vibratory stress in operation of the dynamoelectric machine.
A common disadvantage of the various approaches described above is that, by virtue of the plastic deformation, the resistance of the end turn in the vicinity of the temperature sensor will be changed due to "working of the wire" caused by bending of the wire and/or by tension being applied to the wire during insertion of the temperature sensing device into the end turn, or during the subsequent compressing and compacting of the end turns of the stator after the temperature sensor has been placed therein. This increase in resistance may subsequently lead to the development of a hot spot in the area of the wire which has been plastically deformed, which can eventually cause a failure of the wire, or the insulation system in the vicinity of the temperature sensitive device, particularly where end turns are tightly packed, as a result of precision winding, or where the wires are subjected to high current densities.
Where an electric machine includes a significant thermal mass and operates at a relatively low current density, such as may be the case in an industrial or commercial dynamoelectric machine, the various approaches described above may be adequate to prevent the generation of detectable levels of smoke, gas, or odors, by virtue of the fact that the rate of temperature increase for such a machine under abnormal operating conditions may be slow enough to allow utilization of a means for protecting the electrical machine having a relatively slow response time.
Unfortunately, even where the problems defined above have been overcome, in those applications where the electrical machine must be extremely compact, lightweight (i.e. have minimal thermal mass), and operate at high current densities, as is typical of dynamoelectric machines in aerospace applications, it is believed that the prior art has heretofore failed to produce a thermal protection means which is capable of responding quickly enough to prevent the generation of smoke or odor detectable by passengers or crew members in an aircraft when an abnormal condition such as a locked rotor is encountered.
What is needed then is a compact, lightweight dynamoelectric machine capable of operation with high current densities and having a stator, including a temperature sensor installed in the stator in a manner which provides enhanced thermal conduction resulting in a response to abnormal temperature which is sufficiently fast to prevent the formation of smoke or odor detectable by passengers on an aircraft which may be installed into the stator in a manner not requiring plastic deformation of the end turns.